A transceiver is a well-known circuit, containing a transmitter and a receiver, which are thus capable of transmitting and receiving communication signals, respectively. Conventionally, the transmitter contains a power amplifier (PA) that provides the last stage of amplification of the signal to be transmitted.
In most conventional designs, the power amplifier is implemented as a component that is physically separate from other parts of the transmitter and/or transceiver. Also, power amplifier's made from gallium arsenide (GaAs) or Silicon bipolar junction transistors (SiBJT) are typically used because they have an inherently higher breakdown voltage than transistors made in a CMOS circuit, whether the transistors are n-channel or p-channel transistors. While such designs allow for a power amplifier that has the desired amplification characteristics, they do so at the expense of cost. Not only is a GaAs, SiBJT or other non-CMOS power amplifier costlier than a transistor in a CMOS integrated circuit, but the non-CMOS power amplifier cannot be formed on the same integrated circuit chip as the components of the transmitter and/or transceiver. Both of these factors add to the overall cost of the resulting transceiver.
It has been recognized that it would be beneficial to have a transceiver in which most of the transmitter and receiver circuits are on a single chip, including the power amplifier. For example, in the article entitled A Single Chip CMOS Direct-Conversion Transceiver for 900 MHz Spread Spectrum Digital Cordless Phones by T. Cho et al. that was presented at the 1999 IEEE International Solid State Circuits Conference, there is described a CMOS transceiver chip that includes an integrated power amplifier. An improved CMOS power amplifier is also described in the application entitled CMOS TRANSCEIVER HAVING AN INTEGRATED POWER AMPLIFIER, bearing application Ser. No. 09/663,101, filed on Sep., 15, 2000 and assigned to the same assignee as the assignee of the invention described herein, which recognizes the advantage of integrating the power amplifier.
Nevertheless, a major disadvantage of CMOS power amplifiers is that they exhibit a wide range of power levels variation due to their sensitivity to thermal and process variations. High efficiency and constant power levels in CMOS power amplifiers is impeded by the technologies low breakdown voltage, low current drive, and lossy substrate.
Furthermore, conventional transmitter designs operate so that the output power is transmitted based upon a function of many different variables. In a Code Division Multiple Access (CDMA) environment, for example, the power output of a mobile transmitter will typically be based upon the distance between the mobile transmitter and the base station currently in use. In such an environment, the output power will increase, for example, if the mobile transmitter travels closer to the base station. In operation, the gain of a variable gain amplifier that is part of the transmitter, at either the intermediate frequency (IF) or radio frequency (RF) stage, will be changed to thereby lower the transmit output power. In this situation, while the output power may become too large for a period of time, that is acceptable within the overall system requirements.
In other environments, however, it is required, by for instance the Federal Communication Commission (FCC), that the output power must not exceed a pre-specified level at any time. In such an environment, the above-described design cannot be used. Since in order to take into account instances in which power will exceed the pre-specified maximum, the average output power must be much lower than that maximum, which degrades system performance to an unacceptable level.
Accordingly, a transmitter containing a variable gain amplifier and a power amplifier integrated with a CMOS transceiver chip that overcomes the above disadvantage would be desirable.